Different types of UPS Bypass


Most users do not appreciate that there are several different kinds of UPS bypass. This note describes some of the variations and highlights a common application problem.

The variations

Line interactive UPS usually just sit and wait for the mains power to go away and when it happens switch the critical load to the battery supply. This means that overloads are supplied by the mains.

On more sophisticated products the critical load is supported by the battery and an internal bypass arrangement connects the load to the mains for switch-on overload problems.

Advance also recommends that an extra external manual bypass switch be installed to permit operation of the critical load during UPS maintenance.

On large UPS items (say 5kVA and above) it is quite common to find a maintenance bypass built into the unit. This is used for testing and permits some system flexibility when doing battery tests etc. However we still recommend the use of an external bypass.

Application difficulties

If the UPS is to automatically switch the critical load from bypass mains to battery supply then the two sources must be in phase. If the mains supply has been interrupted the battery supply cannot know how to synchronise with the mains because it is not present. If the maintenace bypass switch is operated during battery operation the critical load may see a supply which in the worst case can see a complete phase reversal. This is almost guaranteed to cause the load to be disrupted.

The solution

This problem is solved by ensuring that only trained personnel operate a manual bypass switch. Advance always labels manual bypass switch boxes with a warning not to operate it when the mains has failed.

It is important that responsible users differentiate between normal battery operation where the inverter and mains are synchronised and the special case where the UPS is providing stand alone power.

Improving reliability in plug 'n play installations

Almost all of the newer plug 'n play bypass boxes permit the entire UPS and maintenance bypass switch to be operated from a single outlet. We recommend that consideration is given to connecting the UPS mains supply and maintenance bypass switch to different in phase supplies. Ideally this is just a double 13A socket with one feeding the UPS and the other the bypass switch. With this arrangement the critical load is no longer dependent on a single fuse. See also our pages on mission critical mission crucial and bypass schematics